Disclaimer 1: Most of the stuff below is fairly heavy on the math side; I haven't found a nice introduction yet (if I keep on reading papers at my current speed, I might as well write one myself - unless BillSpight beats me to it once again). However
http://mathworld.wolfram.com/GameTheory.html might provide a nice jump-off point, but it mostly deals with Classical Game Theory.
Anyway, here are some places to look for stuff on Combinatorial Game Theory.
Online papers and other material:
Offline books:
Disclaimer: I haven't read any of the last three books, but they seem pretty interesting to me...
Author: JanDeWit